There seems to be a bit of confusion when it comes to selecting tires to replace the size/type that came as Original Equipment on your RV. What you can do and what you should do are not always the same thing.
Here are some statements collected and posted by a knowledgeable person who frequently responds to questions on some RV Forums.
"Goodyear: Never fit tires to a vehicle that have less load carrying capacity than required by the Original Equipment Manufacturer.
Michelin: Never choose a tire that is smaller in size or has less load-carrying capacity than the tire that came with the vehicle.
Cooper: The new tires must have a load carrying capacity equal to or greater than the maximum load carrying capacity specified on the tire placard on the vehicle.
Toyo: Any replacement tire must be of a size and load range that will offer equal or higher load carrying capacity compared to the original equipment (OE) tire on the vehicle."
I am not aware of any "legal" requirement that specifies what the RV owner is required to do. While it is a legal requirement from DOT that the vehicle manufacturer must select and specify tires and the inflation necessary to support the stated Gross Axle Weight Rating, as far as I know, this legal requirement does not apply to individual owners of vehicles.
HOWEVER, I doubt that you will find any company or responsible individual willing to state that it is good practice to select replacement tires that do not have a stated load capacity that is equal to or greater than the load capacity of the original tires.
All tires sold for use on public highways have a stated maximum load capacity at a given inflation pressure molded onto both sidewalls. This fact, along with the information in published Load/Inflation tables, makes it relatively easy to find tires with the needed load capacity.
There are a number of reasons to consider an alternate size tire (availability, cost, brand reputation, etc.) but in every case you should only select new tires that can support at least as much load as the OE tires.
The above is based on an assumption that the load on your tires is split equally 50/50 side to side on each axle. Since this is seldom the case, this is an extra bit of information you should consider when shopping for new tires.
I have numerous posts on this blog where I outline the importance of confirming the actual load on each end of each axle. This is because it is possible to have the load unbalanced to the point that one tire may be overloaded even if the total tire load capacity for all the tires on an axle numerically exceeds the total axle load measured on a truck scale.